How to do Prime and Composite Numbers
Composite numbers are number that have more than one factor pair.
24 is an example: 4x6=24, 8x3=24, and 2x12=24. 24 is a composite number because there is more than one set of numbers that can be multiplied together to make 24. Composite numbers must have more than one factor pair. If they don't they are prime.
3 is an example. 1x3=3 Three is a prime number because only one factor pair makes three.
Prime_diagram.png

If you want to know if a number is prime or composite there is an easy way to check. All of the even numbers, except for 2, are composite. This means that if the number you are checking is not two and it is even then it is composite. If it is odd then we have to check it out more carefully.
Prime_1.png
Prime_2.png
Most prime numbers are odd, but not all of them. 3x3=9 so nine is a composite number. Here are some steps to help you decide which odd numbers are composite. If the number is odd you do not need to check for any even divisibility so you can forget about checking for 2, 4, 6, 8, and 10. That leaves only 3, 5, 7, and 9 to check. Three is easy, just add up all the digits in the number and if they equal a multiple of 3 you know the number is composite. While you have the sum of the digits you can also check for nine. If the sum is a multiple of nine then it is composite. If you know the nines finger rule you can check it that way also.
Prime_3.png
Prime_4.png
The next test is 5, which is also easy, of the number you are checking ends in 5 then it is composite.
Prime_5.png
Prime_6.png
Last is the check for seven. Double the last digit and subtract it from the digits that are left. If the answer is a multiple of 7 then the number is composite. This rule is tricky, if you need help with it click here.
Prime_7.png
Prime_8.png
Once you have worked through all this you can guarantee you will know id the number is prime or composite.

Home Page
AAA Math
Prime Number Calculator